For pulse-stimulation, HeLa cells were treated with phorbol 12-myristate 13-acetate (PMA; Sigma-Aldrich, St Louis, MO, USA) (20 ng/ml) for 5 min, or tumor necrosis factor-α (TNF-α; Sigma-Aldrich) (20 ng/ml) for 30 min, washed twice in complete culture medium and then returned to culture.